As Captain America: Civil War started filming, the big question on everyone's mind has been: what causes the epic battle between Cap and Tony? Well, the events in Avengers: Age of Ultron may answer that question.
In the comics, it is the New Warriors that cause a massive explosion that kills hundreds of people that sparks the superhero registration, and ultimately, Civil War. In the MCU, no such event has happened, and probably will not happen before Civil War, right? Or has it?
Now, whether you saw the movie or not, you're familiar with the Hulk vs. Hulkbuster fight. This fight resulted in the destruction of a city in Africa, and in the beginning of Captain America: Civil War, it looks as though our New Avengers are helping restore the battle damage. What if this fight is what causes the superhero registration act to be put into motion?